This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Deamidation may occur as an artifact during sample processing, particularly when using typical tryptic digestion protocols. We have developed an 18O labeling approach to monitor these artificial deamidations, where the enzymatic digestion was carried out in 18O labeled water. While in vivo deamidation causes a +1 Da mass shift per deamidation site due to the substitution of the -NH2 group by the -OH group, those occurred during digestion lead to a +3 Da mass shift, with the extra +2 Da arising from the incorporation of one 18O atom. This study has recently been published (Li et al., 2008) in the Journal of the American Society for Mass Spectrometry, 2008, 19, 855-864.